Generally, an electric rolling stock has a construction to receive the electric power from a catenary using a power collector and to use the received power and cause an electric power converter such as an inverter apparatus to drive an electric motor so as to run. At the time of applying a brake on the rolling stock car, what is called regenerative braking is employed in which the brake force is obtained by means of a regenerative operation of the electric motor. The regenerative electric power generated at that time is supplied, via a catenary, a third rail or the like, to other power-running vehicles or the loads of the power-running vehicles which are located in the vicinity of the electric rolling stock under consideration, and is consumed therein.
However, in the early hours of the morning, during nighttime, or in a quiet railroad section having few trains running thereon, there occurs a situation where no other vehicle is present in the vicinity of the electric rolling stock under consideration (the regenerative load is insufficient), which leads to a situation where the regenerative electric power generated by regenerative braking is not sufficiently consumed. If the regenerative electric power in the electric rolling stock under consideration exceeds the electric power consumed in other vehicles, then the catenary voltage is increased. This may cause various devices connected to the catenary to be tripped and broken due to overvoltage.
In this regard, an inverter apparatus that is installed in an electric rolling stock includes a voltage detector for detecting the catenary voltage or the like (the overhead wiring voltage or e.g., a filter capacitor voltage on the input side of the inverter apparatus, comparable to the catenary). When the catenary voltage or the like increases above a predetermined value, the inverter apparatus performs a regenerative torque decreasing control in which the regenerative braking force is controlled to restrain the generation of regenerative electric power and to ensure that the catenary voltage or the like does not increase up to or greater than the specified value.
Herein, in the case where the specified input voltage of an inverter apparatus functioning as a second electric-power converting unit is lower than the catenary voltage (for example, specification of 600 V), a main circuit is assumed to be configured in which a converter apparatus functioning as a first electric-power converting unit is connected to the input side of the inverter apparatus and is used to supply a voltage to the inverter apparatus with performing step-down control or constant voltage control of the catenary voltage.
For example, for a system having a nominal value of the catenary voltage of 1500 volts DC, a configuration is known in which a converter apparatus steps down the output voltage thereof to about 600 VDC and performs constant voltage control, before inputting that voltage to the inverter apparatus (for example, Patent Literature 1 mentioned below).
In a system having such a main circuit configuration, even when the regenerative load is insufficient thereby leading to an increase in the catenary voltage, the constant voltage control is performed so as to maintain the output voltage of the converter apparatus at a constant voltage. In this way, the input voltage of the inverter apparatus does not vary and the regenerative torque decreasing control is not initiated. For that reason, the regenerative electric power becomes surplus thereby leading to increase in the input voltage of the converter apparatus and to result in the input to the converter apparatus being overvoltage. Thus, the converter apparatus may detect the overvoltage and trip, and/or devices connected to the catenary may be damaged.